The present invention relates in general to pulverized coal supply arrangements, and in particular, to a new and useful flow control system for accurately controlling the mass flow rate of pneumatically transported pulverized coal.
In a pulverized coal burning boiler, one or more pulverizers are used to grind lumps of crushed coal into particulates with a certain desired sized distribution. The ariborne pulverized coal (PC) is transported to each burner in pipes ranging from 8 to 24 inches in diameter. There may be six or more pipes in parallel carrying PC to the burners. Boilers run at high efficiency when the burners are well balanced. Balanced burners require that both the primary air and the mass flow rate of PC are the same among all the pipes within certain operating limits. Each pipeline installed between the pulverizer and the burner generally has hydraulic resistance which is somewhat different from the other lines due to the difference in overall length of the line and the type and number of bends used between the two points. These variations in line resistance cause an imbalance of the primary air flow amoung the PC feed lines if not corrected.
A common industry practice is to add a fixed resistance orifice or pipe in the line that has a lower resistance than desired (FIG. 1). Then the primary air flow in each line is confirmed with a pitot tube in the absence of PC flow. However, the balanced primary air flow alone does not induce a balanced PC flow in the system due to the asymmetric flow distribution at the pulverizer outlet and the peculiarities in the airborne solids flow. Plant operators have reported an excess of 10% deviation in PC flow from the average in a system that had been balanced for the primary air flow using fixed resistance orifices and pipes.
While there are a number of PC flowmeters at various stages of development in the industry, there has not been any commercially available flow control system for P.C. transport lines. The primary reason for the absence of such a system is because it is very difficult to design a reliable control element that can meet a set of very tough operational requirements; namely:
1. For long-term, reliable service, the control element must be highly erosion-resistant if it is to be exposed to flowing coal particles.
2. The element must not appreciably increase the pressure drop of the line; the maximum tolerable increase in pressure drop would vary from plant to plant, but the permissible increase is very small in general;
3. The element should not interfere with the normal flow of the primary air which is required to keep the PC particles airborne;
4. Control should be sensitive enough to effect changes as small as 1-2% in PC mass flow rate; and
5. The element should be energy efficient and retrofittable to be commercially attractive and viable.
Some forms of metallic diverter vanes have been used at some plants on a trial basis. In general, they disrupt the air flow such that saltation is induced in the lines. The vanes are also very short-lived due to the erosion caused by the impinging coal particles flowing at high velocity. There is no other successful or unsuccessful system known, commercial or otherwise, that has been used to change the mass flow rate of PC that is pneumatically transported in a pipe.